Communication systems take many forms. In general, the purpose of a communication system is to transmit information-bearing signals from a source, located at one point, to a user destination, located at another point some distance away. A communication system generally consists of three basic components: transmitter, channel, and receiver. The transmitter has the function of processing the message signal into a form suitable for transmission over the channel. This processing of the message signal is typically referred to as modulation. The function of the channel is to provide a physical connection between the transmitter output and the receiver input. The function of the receiver is to process the received signal so as to produce an estimate of the original message signal. This processing of the received signal is referred to as demodulation and detection.
Two types of two-way communication channels exist, namely, point-to point channels and point-to-multipoint channels. Examples of point-to-point channels include wirelines (e.g., local telephone transmission), microwave links, and optical fibers. In contrast, point-to-multipoint channels provide a capability where many remote stations may communicate simultaneously with a single central communication site (e.g. cellular radio telephone communication systems). These point-to-multipoint systems are also termed Multiple Address Systems (MAS).
The use of modulation permits multiplexing, that is, the simultaneous transmission of signals from several message sources over a common channel. Also, modulation may be used to convert the message signal into a form less susceptible to noise and interference.
For multiplexed communication systems, the system typically consists of many remote units (i.e. mobile communication units) which require active service over a communication channel for a short or discrete portion of the communication channel resource rather than continuous use of the resources on a communication channel. Therefore, communication systems have been designed to incorporate the characteristic of communicating with many remote units for brief intervals on the same communication channel. These systems are termed multiple access communication systems.
One type of multiple access system is a cellular radio communication system. Cellular radio communication systems typically include a number of central communication base sites. Each central communication site has a service area coverage for servicing mobile communication units within the service area. The service areas typically are arranged such that adjacent central communication site service coverage areas overlap in a manner that provides a substantially continuous service region. The substantially continuous service region provides uninterrupted service by handing off mobile communication units from one central communication site serving a service area to an adjacent central communication site serving another service area.
Communication between the central communication sites and mobile communication units typically occurs on a pair of communication resources or channels (i.e., transmit and receive channels) assigned according to a cellular communication system communication resource reuse plan.
To limit noise in cellular communication systems due to communication between other mobile communication units in other nearby service areas serviced by the central communication sites as well as increase the capacity of the cellular communication system, reuse of the available, but limited number of communication resources is done within a service region of the cellular communication system. In the past, in order to ensure that the reuse of communication resources does not cause unacceptable noise in the communication channel, service regions of central communication sites which have been allocated the same communication resources have been geographically separated. By having sufficient geographic separation, a particular type of noise (i.e. co-channel interference) in the communication channel is limited. However, the geographic separation needs to ensure an adequate signal to noise ratio (negligible noise in the communication channel) limits the capacity of the communication system because not all of the available communication resources may be used in each service area.
To enhance the efficiency of communication resource reuse and to improve capacity of the cellular communication system, service areas of central communication sites can be divided into sectors, wherein within each sector all or part of the available communication resources, (i.e., communication channels) is allocated. By having the service area divided into sectors, the required geographic separation may be reduced while maintaining an adequate signal to noise ratio. For example, U.S. Pat. No. 4,128,740, assigned to Motorola, Inc. discloses a four cell (service area)--six sector communication resource reuse pattern. As disclosed, each cell is divided into six sectors and each sector contains approximately 1/24th of the available communication resources. For every four cell sites, the communication resource pattern is repeated. This communication resource reuse pattern may be further reduced to a 1 cell site reuse pattern as disclosed in pending U.S. patent application 07/459,624 by Comroe et al. which was filed Jan. 2, 1990 and also assigned to Motorola, Inc. It will be appreciated by those skilled in the art many other reuse patterns exist for use in cellular communication systems including but not limited to 3, 7, 21, 49, 63, 91 site reuse patterns.
In addition, several reuse patterns have also been used simultaneously in cellular communication systems. U.S. Pat. No. 4,144,411 issued to Frenkiel on Mar. 13, 1979, teaches one such cellular system in which static reuse of frequencies in a large-cell frequency reuse pattern an in a miniature-sized overlaid, but same type frequency reuse pattern. The miniature-sized reuse pattern and the large-cell patterns are both on seven-cell reuse patterns. This is achieved through yet lower transmit powers and maintaining the same site spacing to cell radius as the large-cell. This concept is one method of improving frequency reuse of traffic channels in a geographic region. The decision to handoff from an overlaid serving cell to an underlaid cell is based on whether a subscriber's received signal strength (RSS) is greater than a threshold set for the overlaid cell. If the RSS is equal to or less than the predetermined threshold, a check is made to see if there is a large-cell channel available.
An enhancement to Frenkiel is discussed in an article authored by Samuel W. Halpern entitled "Reuse Partitioning in Cellular Systems", presented at the 33rd IEEE Vehicular Technology Conference on May 25-27, 1983 in Toronto, Ontario, Canada. The Halpern article sets forth a cellular system having multiple reuse levels (or patterns) within a given geographical area. A reuse level refers to a particular channel reuse pattern whether the channel is based on frequency, time slots, codes, or other suitable divisions. For example, a cluster of cells normally employing a seven-cell reuse pattern may simultaneously operate on a three-cell reuse pattern whereby one set of frequencies is dedicated to the three-cell reuse pattern while another set of frequencies is dedicated to the nine-cell reuse pattern. This division of frequency spectrum into two groups of mutually exclusive channels is one method of providing multiple reuse patterns. Consequently, one cell site may operate on both a nine-cell and a three-cell reuse pattern by using channels from its channel set which are dedicated to specific cell sites and assigned to the different reuse patterns. Such smaller reuse patterns form a noncontiguous overlay of cells having a decreased radius.
It will be appreciated by those skilled in the art that communication resources or channels may be distinct frequencies (i.e., a frequency-division multiple access (FDMA) communication system). However, other types of communication resources or channels exist which may be assigned according to a communication reuse separation pattern. These other communication resources include data channels and traffic channels, which may be time slots within the same frequency such as in a time division multiple access (TDMA) system, or traffic channels and data channels in other types of channelized systems such as code division multiple access (CDMA) systems.
One type of communication system which can be a CDMA system is a spread spectrum system. Three general types of spread spectrum communication techniques exist, including: